Effect of backing thickness on determination of the phase in neutron reflectometry by variation of backing

TL;DR

This paper investigates how the thickness of the backing material affects the accuracy of phase determination in neutron reflectometry, revealing that assuming an infinitely thick backing can lead to incorrect results for weakly absorbing substrates.

Contribution

It demonstrates that considering finite backing thickness is crucial for accurate phase determination, challenging the common assumption of semi-infinite substrates in neutron reflectometry.

Findings

Neglecting backing thickness can cause significant errors in phase determination.

The method of variation of the backing is unreliable for weakly absorbing substrates.

Finite backing effects must be included for precise neutron reflectometry analysis.

Abstract

The determination of density profiles with knowing the phase information of complex reflection coefficient for neutron specularly reflected from a film, yields unique results. Recently it has been shown that the phase can be determined by using controlled variation of the scattering length density of the fronting (incident) and/or backing (substrate) medium instead of reference layers of finite thickness. This method is applicable under the simplifying assumption that the backing is infinitely thick (semi-infinite substrate). By this assumption the reflected beams from the end side of the backing is neglected, which appears reasonable since in most cases absorption will damp out the neutron current before it has reached the end side. But for weakly absorbing backing, the reflection from the end side may be considerable. So backing must be considered as a thick matter. We show that for this kind of backing, using method of variation of the backing, as an example of variation of the surroundings, leads to completely wrong answer in determination of the phase.